It is estimated that approximately one in eight women will be diagnosed with breast cancer at some point during their lifetimes. The evidence for an association between physical activity and reduced risk of breast cancer is convincing, however, the optimal amount of physical activity for reducing breast cancer risk, in terms of intensity, duration, and frequency, as well as the mechanism by which exercise produces these beneficial effects, remains unclear. Our research group has shown preliminary correlational data linking increased physical activity to decreased DNA methylation-a potential biomarker for cancer risk-among healthy adults. The causal effect of exercise-and in particular exercise volume, intensity, and duration-on methylation needs to be explored in a carefully controlled experimental design. Further, no research has been done on the duration of exercise-induced methylation changes, which is another critical issue with respect to the public health implications of exercise recommendations. Thus while there is a clear epidemiological link between physical activity and reduced risk of developing breast cancer, and increasing evidence for the influence of physical activity on DNA methylation effects on breast cancer development, the next step is to better understand the dose response relationship of exercise on the epigenetic modifications that may ultimately put a woman at risk for breast cancer. The purpose of the proposed study is to explore the influence of exercise on preclinical and easily obtained biomarkers of DNA methylation that are potentially associated with risk for the development of breast cancer. We hope to demonstrate a dose response relationship in exercise volume controlling for both intensity and duration of exercise, such that increases in exercise volume will be associated with greater changes in DNA methylation, providing evidence of a mechanistic and causal link between behavior (participation in aerobic exercise) and epigenetic changes in DNA that are likely to be associated with breast cancer risk. We will focus this application on sedentary women, a group at higher risk for the development of breast cancer than regularly exercising women, and on DNA methylation on the CpG islands of genes associated with breast cancer, as in our previous work.